1. Technical Field
The present invention relates to a vibration type of micro gyro sensor equipped with two vibrators.
2. Related Art
In general, a gyro sensor has been used in various applications such as attitude control system. The gyro sensor is classified into many types including a vibration type of micro gyro sensor equipped with a vibrator.
For driving the vibration type of gyro sensor, it is general that the vibrator is forced to vibrate at a resonance frequency thereof. The reason lies in improving sensitivity to sense angular velocity. When being vibrated at the resonance frequency, the vibrator shows a vibrational amplitude multiplied by a Q-value of the vibrator, thus raising a vibration velocity of the vibrator depending on the Q-value. This rise in the vibration velocity will improve the sensitivity for angular velocity.
A linear vibration type of micro gyro sensor falls into the vibration type of micro gyro sensor. In such a linear sensor, two vibrators are generally used. Specifically, to reduce an anti acceleration sensitivity, the two vibrators are driven to vibrate in mutually opposite phases and two acceleration signals from the two vibrators are subjected to mutual subtraction between those two acceleration signals, thus canceling an acceleration component. To gain this canceling effect, the two vibrators should be vibrated at the same vibration frequency.
In FIG. 1, a conventional two-vibrator type of micro gyro sensor 1, which is categorized in the linear vibration type of micro gyro sensor, is exemplified. The gyro sensor 1 shown in FIG. 1 includes two vibrators 11 and 21, each of which is supported by driving beams 12 (22) so that each vibrator can easily vibrate in an X-axis direction defined in FIG. 1. To each vibrator 11 (21), there are formed monitoring electrodes 13 (23) to monitor the vibration of the vibrator 11 (21) and movable electrodes 11a (21a) to be opposed to driving electrodes 14 and 15 (24 and 25) in a comb-shaped form. References 16a, 16b and 26a and 26b shown in FIG. 1 are detecting electrodes for detecting a signal from the sensor 1.
To both of the driving electrodes 14 of one vibrator 11 and the driving electrodes 25 of the other vibrator 21, a driving signal of which frequency is equal to the resonance frequency is applied by a self-energizing oscillator 30. In addition, to the driving electrodes 15 of one vibrator 11 and the driving electrodes 24 of the other vibrator 21, the opposite-phase driving signal to the above driving signal is applied through an inverter 31 by the self-energizing oscillator 30.
When each of the vibrators 11 and 21 vibrates in the X-axis direction, an opposed area S formed among the electrodes of each vibrator 11 (21) varies, thereby a capacitor C between the electrodes varies responsively based on the relationship of C∝∈·S/d (∈; dielectric constant, d; distance between the electrodes). The vibrations of the vibrators 11 and 21 are monitored by the monitoring electrodes 13 and 23, respectively, to subtract monitored signals one from the other by a differential amplifier 32. A resultant subtracted signal is fed back to the self-energizing oscillator 30 to drive both the vibrators 11 and 21 at the same frequency signal (but the signals applied to both the vibrators 11 and 21 are mutually opposite in their phases, thanks to the inverter 31 intervening in the path to the other vibrator 21).
However, the above two-vibrator type of micro gyro sensor is still unsatisfactory in its performance. Manufacturing such a sensor frequently involves irregularities in sizes or others. Such irregularities, which include a quantity of mass of the vibrators and sizes of the driving beams, often cause a difference in the resonance frequencies of the vibrators 11 and 21. When such a difference is caused, applying the signal resultant from the addition of the monitoring signals from the two vibrators 11 and 21 to both the vibrators 11 and 21 will result in that the two vibrators 11 and 21 are forced to self-oscillate at a signal of which frequency lies between the two resonance frequencies given to the two vibrators 11 and 21.
On top of this, in a situation where the Q-value of each vibrator is high to make it larger the difference between the two resonance frequencies, the two vibrators 11 and 21 are forcibly driven at a smaller amplitude when receiving the driving signal whose frequency lies between the two resonance frequencies. As a result, in such cases, the two vibrators cannot be self-oscillated.